B. OXIDATION NUMBERS (EQUIVALENT TO VALENCIES)
The oxidation number is the “apparent” charge of an atom in a compound or ion. The bond may be covalent. eg. CH4(g).
An accountant is used to check the books to evaluate cash inflows and outflows in business. Oxidation numbers are used to check the books in a chemical reaction.
1. The oxidation number of all elements is 0. (In any allotropic form).
Element |
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Oxidation number |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
2. The oxidation numbers of the elements in Groups I and II are always +1 and +2 respectively.
The oxidation number of Group III elements are nearly always +3. i.e. valency of metals = the oxidation number
Chemical species |
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|
Oxidation number of atom in bold font |
+1 |
+2 |
+3 |
+1 |
+1 |
+3 |
+2 |
+1 |
+2 |
+1 |
3. The oxidation number of hydrogen is +1 in all its compounds except with metal hydrides, where it is -1.
Chemical species |
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|
Oxidation number of hydrogen |
+1 |
+1 |
+1 |
+1 |
-1 |
+1 |
+1 |
-1 |
-1 |
-1 |
4. The oxidation number of oxygen is -2 in all its compounds except peroxides where it is -1 and with fluorine where it is +2, since fluorine is the single element more electronegative than oxygen).
Note: The oxidation number of fluorine is always -1, since it is the most electronegative element.
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Sodium perioxide |
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Hydrogen peroxide |
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Chemical species |
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Oxidation number of atom in bold font |
-2 |
-2 |
-2 |
-2 |
-2 |
-2 |
-1 |
-2 |
-1 |
+2 |
5. The oxidation number of monatomic ions = ionic charge
Chemical species |
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Oxidation number of atom in bold font |
-2 |
-2 |
+2 |
+3 |
+2 |
+2 |
-1 |
+3 |
+2 |
+3 |
6. All other oxidation numbers of atoms in a chemical species are calculated so as to make the algebraic sum equal to the net charge of the molecule or ion.
Sum of oxidation numbers of ALL atoms = Net charge of the molecule or ion
Chemical species |
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Net charge of species |
0 |
-1 |
-2 |
0 |
-3 |
0 |
-1 |
-2 |
+1 |
-2 |
The oxidation numbers of the atoms in the above table will be determined in the part C which will be published soon..